Protecting Carbon Nanotubes From Oxidation for Selective Carbon

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C: Physical Processes in Nanomaterials and Nanostructures

Protecting Carbon Nanotubes From Oxidation for Selective Carbon Impurity Elimination Nawal Berrada, Alexandre Desforges, Christine Bellouard, Emmanuel Flahaut, Jérôme Gleize, Jaafar Ghanbaja, and Brigitte Vigolo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12554 • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 27, 2019

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The Journal of Physical Chemistry

Protecting Carbon Nanotubes From Oxidation for Selective Carbon Impurity Elimination

Nawal Berrada1, Alexandre Desforges1, Christine Bellouard1, Emmanuel Flahaut2, Jérôme Gleize3, Jaafar Ghanbaja1, Brigitte Vigolo1,*

1Institut

Jean Lamour, CNRS-Université de Lorraine UMR 7198, Campus Artem, 2 allée André

Guinier, 54011 Nancy, France 2CIRIMAT,

Université de Toulouse, CNRS, INPT, UPS, UMR CNRS-UPS-INP 5085,

Université Toulouse 3 Paul Sabatier, Bât. CIRIMAT, 118, Route de Narbonne, 31062, Toulouse Cedex 9, France 3Laboratoire

de Chimie Physique-Approche Multi-échelle de Milieux Complexes-University

of Lorraine, 1 Bd Arago, 57078 Metz, France

*

Corresponding author. Tel: +33 372 742 533, E-mail: [email protected] (B. Vigolo) 1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Purity of carbon nanotubes (CNTs) is essential to avoid a dramatic decrease in their performances. In addition to metallic impurities, carbonaceous impurities have been shown to be responsible for pronounced effects. However, they are highly difficult to be selectively removed from CNT samples because of the similar chemical reactivity of these two kinds of carbon species. The existing purification methods often lead to high CNT consumption (> 90 wt.%). The proposed method consists of a one-pot gas phase treatment combining chlorine and oxygen. The CNT powder maintained in a chlorine stream is submitted to oxygen at moderate temperature (350°C and 500°C for SWCNTs and DWCNTs, respectively) and the thermal treatment is then pursued at 900-1000°C under chlorine alone. Our work reveals that this approach is able to significantly improve the selectivity of elimination of carbonaceous impurities. Thanks to the proposed purification treatment, only 19 and 11 wt.% of carbon species (mainly carbon impurities) are lost for DWCNTs and SWCNTs, respectively. The mechanism proposed involves a protective effect by grafting of chlorine favored to the CNT walls. Because our simple one-pot purification method is also versatile and scalable, it opens new perspectives for CNT applications in high-added value fields.

Keywords: Carbon nanotubes; purification; carbon impurities; selectivity; gas phase; quality

2 ACS Paragon Plus Environment

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1. Introduction Carbon nanomaterials, such as carbon nanotubes (CNTs) and graphene, are exciting materials showing a combination of superior properties: lightness, thermal and electron conductivity, optical and mechanical properties.1 The technology of CNT synthesis, while in a continuous state of improvement, cannot avoid the use of catalysts to increase the conversion rate from carbon precursors to the production of carbon nanomaterials, as this rate is still far below 100 %.2–6 Presence of both metal and carbon impurities have negative effects on the CNT properties.7,8 It is widely known that magnetic properties of CNTs are dominated by catalyst residues even at low content.9 Thermal and electrical conductivity as well as mechanical properties (tensile strength) are dramatically decreased because of carbon impurities in CNT samples.10 Although it was firstly attributed to the CNTs themselves, the observed electrocatalytic activity has been shown to be due to metallic catalyst impurities11,12 or to carbonaceous by-products13–15. Similar mistrusted effects on electrochemical properties from carbon impurities in graphene samples have been reported by Pumera et al..16 Purification of CNTs has been extensively desired and various methods allow to efficiently remove metallic contamination.17,18 Some methods to remove only catalyst residues consist of annealing the CNT powder at temperature in the 2000-2500°C range under N2, Ar or vacuum.19– 21

However, even if high-quality CNTs can be prepared from the standard methods, they still

fail in selectively removing carbonaceous impurities without excessive loss of CNTs leading to low sample yield.22 The main reason explaining this difficulty to eliminate carbon impurities without attacking the CNTs comes from their too close chemical reactivity.23,24 Standard



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approaches use strong acids which are known to damage the CNTs and lead to weak sample yield; they create as well functional groups at the CNT surface and produce large amounts of amorphous carbon debris.25 These surface groups and debris are prejudicial for further CNT applications for which surface and interfacial phenomena are playing the major role.8,26 Gas phase routes using air and/or oxygen, hydrogen, carbon dioxide and ammonia have been also reported as efficient purification methods to selectively remove carbonaceous impurities from CNT samples.27,28 Thermal air or oxygen oxidation at moderate temperature was able to selectively attack amorphous carbon or carbonaceous impurities of lower oxidation resistance than CNTs.29–32 High temperature heating in hydrogen or in ammonia has been proposed as effective purification treatments.33,34 Carbon dioxide was also used as a milder oxidant with successful attack of carbon impurities and less damaging of CNTs.35,36 For these gas-phase methods, a subsequent treatment usually in a liquid medium such as nitric of hydrochloric acid is required to remove the metallic impurities. Interestingly, halogens such as bromine37 and chlorine under various forms (hydrogen chloride38, tetrachloromethane39 or chlorine40,41) are able to effectively remove metallic impurity from CNTs, carbon species (including CNTs and carbonaceous impurities) being untouched. In a previous work42, we had shown that addition of oxygen in chlorine could lead to remove carbon impurities from DWCNTs. Unfortunately, the combustion rate at the high temperature used (950 °C) was too rapid and not possible to be controlled to reduce the CNT loss which was as high as 95 %. In spite of the extensive research in the field, efficient elimination of carbonaceous impurities from CNT samples has not been achieved without damaging and consuming the majority of CNTs; the missing key is to reach high degree of selectivity of carbonaceous impurity attack. We present here experimental results that evidence a high selectivity in elimination of carbonaceous impurities with respect to CNTs occurring in particular working conditions. Moreover, in contrast with the few available reported non-damaging methods, our approach


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allows to prepare purified CNTs while limiting the number of chemical treatments.43–45 Our entirely gas-phase treatment consists of a concomitant thermal activation of a protection/combustion effect of the carbon species before the spontaneous elimination of the formed metallic chlorides in one-pot treatment. The favored oxidation of carbonaceous impurities is demonstrated by applying this treatment to both single-walled CNTs (SWCNTs) or double-walled CNTs (DWCNTs). The results show that the combination of chlorine and oxygen at temperature as low as 350 – 500°C can be a powerful asset for an effective carbon impurity elimination with a much higher carbon yield than previously reported 81 wt.% instead of 5 wt.%42. These results have been achieved thanks to a strong protecting effect of CNTs against combustion occurring in a quite low-temperature range (350-500°C) when oxygen is introduced in chlorine while carbonaceous impurities remain susceptible to be vaporized. The proposed reaction mechanism, unprecedented in the literature, which allows to explain this finding is based on the identified tunable reactivity between oxygen and chlorine towards carbon species, i.e. carbon nanotubes and carbon impurities. The method we propose here fulfills hence all the requirements to purify large volumes of CNTs with high sample yields which is of great interest for their practical applications.

2. Material and methods 2.1. CNT purification. The DWCNT sample was synthesized by CCVD with optimized operating conditions,46 the raw DWCNT sample is referred to as rD. The raw HiPco SWCNT samples used for this work was supplied by NanoIntegris InC. For the purification of the HiPco sample, because of the numerous conditions tested, we had to use several raw batches of SWCNTs. Each one has been distinguished: rS1-2 is the starting raw SWCNT sample which has been purified to prepare S1 and S2; and rS3-5 corresponds to the raw SWCNTs used to



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obtain S3, S4 and S5; and rS6 for S6. The experimental conditions used for the purification of both SWCNTs and DWCNTs are given in Supporting Information, Table S1. Moreover, the treatment of a DWCNT sample was intentionally stopped after the O2 dwell under Cl2 at 500°C, this sample is referred to as mD. For the purification treatment (Fig. 1), the CNT powder (~ 150 mg) was placed in a silica boat (length 10 cm) in a tubular oven (diameter 2 cm) which was first flushed by nitrogen (nitrogen being also the carrier gas for the process) to carefully remove air; purified chlorine at around 200 mL/min was incorporated into the set-up while the temperature was increased (10°C/min). When temperature reached the chosen value 𝑇𝑂2 (𝑇𝑂2 variation range: 250-950°C), O2 with a flow rate of 2-4 mL/min was then injected for the desired duration (t1-t2 in the 20-60 min range). After the Cl2/O2 treatment stage, the sample was heated at 𝑇𝐶𝑙2 for one hour under chlorine alone before natural cooling (see also Table S1). With the used conditions regarding the O2 flow rate and duration of its injection, the used O2 volume is around 200 mL (9.10-3 mol). The amount of carbon for the experiments is at most 8.10-3 mol. That means that for every experiment carried out, the amount of oxygen content used is sufficient enough to burn all the unwanted carbon-based species. 2.2. Characterization techniques. Transmission electron microscopy (TEM) observations were performed using a JEM-ARM 200F apparatus at an accelerating voltage of 80 kV. At least 30-40 images taken at different places were analyzed for each sample in order to guarantee a representative description of the samples. Thermogravimetric analysis (TGA) was performed with a Setaram Setsys evolution 1750 by using dry air as the carrier gas and a temperature ramp of 5°C/min from room temperature to 900°C. The metal oxide content evaluated by TGA was directly used to calculate the yield of removal of metallic impurity (𝑌𝑚) after purification; that means that the oxygen


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part corresponding to the respective stoichiometric amount in the oxidized form of each metal present in the (raw or purified) CNT samples is included in 𝑌𝑚. Micro Raman spectroscopy was carried out with a LabRAM HR 800 micro-Raman spectrometer with two incident wavelengths: 632.8 nm and 514.5 nm. The incident laser beam, focused on the sample with an x50 objective, was reduced with an optic filter to avoid any damaging due to overheating during the spectrum recording. The recorded number of spectra (at least 3) depended on the observed data dispersion. After subtraction of a baseline, the ID/IG intensity ratio was determined by dividing the height of the D band by that of the G band. As only one representative spectrum is shown for each sample, D/G ratio was close to the average of ID/IG of the sample. The given ID/IG evolutions correspond to the evolution of the average value of the ID/IG ratios obtained for each spectrum for the sample, after the applied chemical treatment. For Auger Electron Spectroscopy (AES), the CNT powder is fixed with a copper scotch on a molybdenum sample-holder. A primary electron energy of 2500 eV is focused on the sample and a cylindrical detector with a resolution of 1 eV is used for data collection. XPS spectra were collected on a Kratos Axis Ultra (Kratos Analytical, U.K.) spectrometer equipped with a monochromatic Al Kα source (1486.6 eV). All spectra were recorded at a 90° takeoff angle, with the analyzed area being currently about 0.7 x 0.3 mm. Survey spectra were acquired with 1.0 eV step and 160 eV analyzer pass energy and the high-resolution regions with 0.1 eV step and 20 eV pass energy (instrumental resolution better than 0.5 eV). Curve fitting was performed using a Gaussian/Lorentzian (70/30) peak shape after Shirley’s background subtraction and using X-vision 2.2.11 software.



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3. Results and discussion 3.1. Purification approach, carbonaceous impurity removal and CNT quality From the synthesis method, catalyst residues remain in the CNT samples, including DWCNTs and SWCNTs. DWCNTs contain cobalt- and molybdenum-based impurities. The latter represent about 10 % of the sample weight. The as-produced HiPco SWCNT sample contains iron-based impurities at a non-negligible amount reaching a third or more of the sample weight. The used DWCNTs have an internal and external average diameter of 1.35 nm and 2.05 nm respectively, with a length between 1 and 10 m. The diameter of the SWCNTs varies from 0.7 nm to 2 nm and their length is similar to that of the DWCNTs. Non-nanotube carbon species are also present in both CNT samples, inherently to the CVD synthesis. The close reactivity between these carbonaceous impurities and the CNTs is clearly put into evidence from their resistance against oxidation by means of TGA. Combustion of CNT samples commonly appears as one single weight loss meaning that CNTs and non-nanotube carbon species burn off in the same range of temperature as it is the case also for the DWCNTs and SWCNTs used here (cf. Fig. 2). The DWCNT and SWCNT as-produced powders were submitted to our one-pot thermal treatment under chlorine (until 𝑇𝑚𝑎𝑥) in which oxygen is introduced at a chosen temperature, 𝑇𝑂2(𝑇𝑂2being 𝑇𝑚𝑎𝑥), the carrier gas being nitrogen, N2 (Fig. 1 and Table 1). With the aim of investigating the mechanisms involved in the Cl2/O2-based treatment, we have performed experiments with different approaches i) O2 was introduced in the reactor at a temperature lower than that used for the final dwell in chlorine (𝑇𝑂2 < 𝑇𝑚𝑎𝑥) (D1, S1 and S2); ii) O2 was introduced during the Cl2 dwell, 𝑇𝑂2 = 𝑇𝑚𝑎𝑥 (D2, S3 and S4); iii) treatments under Cl2 only (D3 and S5) or O2 only (D4 and S6) were also conducted (Table 1 and Supporting Information, Table S1). Among all the performed treatments, for the sake of clarity we present those showing significant


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differences in metallic removal and/or carbon impurity elimination; consistency and reproducibility having been carefully tested.

Figure 1 Schematic of the applied one-pot purification method displaying the nature of the gas used in each step, and the temperature sequence (dwells and ramps).

The typical behavior from TGA of the raw and the treated CNT samples after treatment with Cl2 alone and with a mixture Cl2/O2 is shown in Figure 2. For the raw DWCNT sample, the shape of the TGA curves and the observed combustion temperatures are quite similar (around 500°C) before or after purification (Fig. 2a). Raw SWCNTs burn off around 350°C and the combustion temperature is upshifted to around 550°C after purification (Fig. 2b). The pronounced upshift (~ + 200°C) of the combustion temperature for SWCNTs may be due to the greater metal content in this sample compared to that of DWCNT; upshift of the burn off temperature of CNTs is indeed commonly observed after elimination of the metallic impurities47 (Supporting Information, Fig. S1).



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Figure 2 TGA curves (in dry air, temperature ramp 5°C/min) for the raw and selected CNTs treated with Cl2 and Cl2/O2 (a): raw DWCNT rD (blue); Cl2 purified DWCNT D3 (red); Cl2/O2 purified DWCNT D1 (green); and of SWCNTs (b) raw SWCNT (blue) rS1-2; Cl2 purified SWCNT S5 (red); Cl2/O2 purified SWCNT S1 (green).


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Table 1. Temperature at which O2 is introduced in the purification reactor, temperature of Cl2 dwell (𝑇𝑚𝑎𝑥), occurrence of carbon impurity removal, resulting oxidized metallic impurity content 𝑀𝑝 in wt.% and their content 𝑛𝑚 in at.%, metal removal yield 𝑌𝑚 and carbon consumption after purification 𝐶𝑐 . 𝑌𝑚 and 𝐶𝑐 are defined in the text. ¥ from an earlier work42

Sample

O2

Cl2

introduction

dwell

temperature (𝑻𝑶𝟐)

temperature 𝒎𝒂𝒙

(𝑻

)

Carbon impurity

𝑴𝒑 (𝑴𝒓 )

removal

/wt.%

𝒏𝒎

𝒀𝒎

𝑪𝒄

/at.%

/%

/%

/°C

/°C

rD

-

-

-

10.2

1.9

-

-

D1

500

1000

y

4.8

0.9

53

19

D2¥

950

950

y

0.01

0.002

99.9

95

D3

-

1100

n

5.5

0.6

46

21

D4

500

-

n

72.8

85.0

-

98

rS1-2

-

-

-

28.0

7.7

-

-

S1

350

900

y

5.4

1.2

81

11

S2

600

900

y

4.8

1.1

83

47

rS3-5

-

-

-

44.5

14.7

-

-

S3

800

800

y

5.0

1.1

88.8

76

S4

900

900

y

3.6

0.8

91.9

91

S5

-

900

n

5.2

1.2

88.3

3

rS6

-

-

33.0

9.6

-

-



S6

350

-

n

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N.A.

Figure 3 shows TEM images of the used DWCNT and SWCNT samples before (Fig. 3, rD and rS) and after different treatment conditions. Metallic and carbon impurities can be easily noticed in both SWCNT and DWCNT samples on the TEM images of the starting raw samples. As normally observed, these impurities widely cover the CNT interlacing. After heating the powdered CNT sample under chlorine, carbon impurities were still present and could be clearly imaged (Fig. 3, D3 and S5). These non-nanotube species are present under various shapes, more or less spherical or crooked particles, with a size in the 5-10 nm range or higher. At the nanometer scale, their structure is quite well-ordered with graphitic multi-layer walls observable at high magnification (not shown). Quantifying the carbonaceous impurities within a CNT sample is complex.48–51 Even if it remains very rough, their content can be estimated from TEM28; about 10-30 % in these samples. As expected and already reported by applying chlorine alone40,41, carbon impurities were not eliminated from the CNT samples (D3 and S5). However, they were completely destroyed as oxygen was added to chlorine (Fig. 3, D1 and S1). In the Cl2/O2 treated samples (D1, D2, S1, S2, S3 and S4), CNTs appear very clean, without any other impurity visible on most of the TEM images (Table 1). From time to time, either a metallic or a carbon impurity could still be observed but the efficiency of their removal is really obvious. In the presence of chlorine, the carbon impurities could be selectively eliminated from the CNT samples when O2 is introduced at 500°C or higher and at 350°C or higher for DWCNTs and SWCNTs, respectively. Absorbance spectroscopy, performed on S1 (corresponding to the SWCNTs purified by using the optimized conditions) has shown that carbon impurity content was significantly reduced in these purified SWCNTs compared to the raw SWCNTs (cf. Supporting Information, Fig. S2). If O2 is introduced at lower temperature,


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the amount of carbon impurities with respect to CNTs is alike to what it is observed in the respective raw sample. When only oxygen was used (D4 and S6), carbon impurities were not eliminated from the samples (not shown) and combustion of both CNTs and carbon impurities occurred simultaneously, as further discussed from TGA.

Figure 3 Typical TEM bright field micrographs of the raw and treated DWCNT and SWCNT samples: raw DWCNT (rD), Cl2 purified DWCNT (D3), Cl2/O2 purified DWCNT (D1), raw SWCNT (rS1-2), Cl2 purified SWCNT (S5), Cl2/O2 purified SWCNT (S1).



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Raman spectroscopy is a powerful technique to probe any modification (improvement or damaging) of the CNT structure after a chemical treatment.52 In a Raman spectrum, the G band (around 1590 cm-1) is characteristic of the sp2 network formed by the double-bonded carbon atoms and the D band (around 1330 cm-1) is related to ‘disorder’ or defects when hybridization of carbon atoms turns to sp3 through damaging or functionalization. Evolution of the ID/IG intensity ratio is hence a signature of structural quality modification of CNTs. Figure 4 gives the variation of the ID/IG intensity ratio for the studied samples including Cl2-, O2- and Cl2/O2purified CNTs for both DWCNT and SWCNT.

Figure 4 Variation of ID/IG (%) of each treated sample compared to its respective raw CNT sample. ¥from a preceding work (ref. 42).

For our treatment conditions, except for S2 with the red incident wavelength ( = 632.8 nm) for which the temperature (600°C) for oxygen introduction can be prejudicial to some part of CNTs, ID/IG was decreased for all the treated samples, meaning that the CNTs were not 14 ACS Paragon Plus Environment

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damaged by any of the applied treatments (Fig. 4). The observed decrease of the D band might be due to various effects which can be also combined: i) curing of CNT defects due to the high used temperature, ii) elimination of the most defective CNTs, iii) removing of amorphous carbon layers deposited on CNTs.53,54

3.2. Impurity removal yield, sample yield and overall efficiency of the purification method TGA in air is commonly used to determine oxidized mineral impurity content in CNT samples. With increasing temperature, complete combustion of the carbon species occurs and the remaining weight corresponds to non-carbon impurities: i.e. oxidized metallic impurities. As usually encountered, the oxidized metallic impurities, 𝑀𝑟 are about 10 wt.% (~ 2 at.%) and 3045 wt.% (~ 7-15 at.%) for the used raw DWCNT and SWCNT samples, respectively (Table 1). The atomic content of metal-based impurities 𝑛𝑚 was roughly estimated by dividing the metal oxide weight (from TGA) by the molar mass of the corresponding catalyst(s). Whatever the CNT purification conditions, in the presence of chlorine, the content of the metal residues (𝑛𝑚) falls down to values close to 1 at. % for the two types of samples (Table 1). Removal of metal impurities certainly occurs in the high temperature range while either Cl2 or Cl2/O2 is used. At these temperatures, metal chlorides are easily formed and their sublimation is favored41 leading to their elimination from the CNT samples. The metal removal yield 𝑌𝑚 defined as 𝑌𝑚 =

𝑀𝑟 ― 𝑀𝑝 𝑀𝑟

, where 𝑀𝑟 and 𝑀𝑝 correspond to the

mineral residue content from TGA; i.e. the oxidized metallic impurities, of the raw and the purified CNT, respectively. 𝑌𝑚 is relatively good (80-90 %) especially for SWCNTs since the raw SWCNT sample contains a large amount of catalyst impurities. That part of remaining metal residues is much more difficult to remove from the samples without destroying the majority of CNTs. It certainly corresponds to the metal nanoparticles already described to be



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highly protected in the CNT inner channels; their encapsulation occurring during CNT growth by CVD5,55–57. For both DWCNT and SWCNT samples, the carbon impurities could be efficiently eliminated when Cl2/O2 was used instead of Cl2 alone (Fig. 2 and Table 1). The second relevant parameter to assess a CNT purification process is the carbon consumption (𝐶𝑐) since it eventually allows to appreciate the CNTs lost through the treatment. 𝐶𝑐 corresponds to the carbon loss through purification: 𝐶𝑐 =

100 ― 𝑀𝑟 ― (100 ― 𝑀𝑝) 𝑌𝑠 100 ― 𝑀𝑟

𝑚𝑝

where 𝑌𝑠 = 𝑚𝑟

is the overall sample yield determined from the ratio between the weight of the purified sample 𝑚𝑝 over the mass of the raw one 𝑚𝑟. We point out that 𝐶𝑐 includes consumption of both CNTs and carbon impurities. As expected, increasing 𝑇𝑂2, which allows a better catalyst removal, also induces high carbon consumption (> 50 %) (D2, S4 and S5). Importantly, this consumption is well below 50 % for D1 and S1, samples in which metallic and carbon impurities have been removed with good yields for both of them.

In the literature, it is problematic to retrieve the overall sample loss or carbon impurity loss because of the complexity of the applied treatments and the multi-step nature of the methods involved. Regarding DWCNTs, Flahaut et al. have reported an effective elimination of disordered carbon species by air53 with a sample yield lower than 20 %31. Selective elimination of carbon impurities from SWCNT samples is found in a few studies. The work by Dementev et al. reported selective removal of carbon impurities from (arc discharge produced) SWCNTs by dynamic air oxidation, but metal impurities were not removed.58 Rosario-Castro et al. used a standard multi-step purification method for purifying HiPco SWCNTs including a nitric acid treatment followed by wet air oxidation and Soxhlet extraction in HCl and finally the sample was annealed at 425°C.59 Iron content was reduced by 89 % and carbon loss subsequent to the applied treatments was not determined. Wang et al. applied a wet combined treatment using


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H2O2 and HCl. They could decrease the iron content in HiPco SWCNTs by 86 % while keeping a carbon yield of 75%, but “little damage” of the CNT sidewalls was reported.44 Among the purification procedures involving on halogen-based compounds, after oxidation of iron impurities by oxygen, Xu et al. used a mixture of fluorinated compounds (SF6 and C2H2F4) to form metal fluorides eliminated by subsequent Soxhlet extraction (multistep method).60 The authors reported a SWCNT yield of 68 % with also “little sidewall damage”. Zimmerman et al. have developed a chlorine-based method consisting of bubbling Cl2 in HCl containing SWCNTs and they lost 96 wt.% of the starting HiPco SWCNTs sample.38 To the best of our knowledge, purification showing a selective elimination of carbon impurities, with low carbon consumption (< 50 %) and without any CNT damage, has not been reported yet. Remarkably, our results show that the method we propose is able to successfully combine metal elimination and selective attack of carbon impurities among carbon species in a one-pot treatment. Structural quality of the purified CNTs is preserved and even improved without an excessive consumption of the nanotubes.

3.3. Proposed mechanism We propose a mechanism explaining the combustion selectivity towards the carbonaceous impurities based on the known specific reactivity between halogens and carbon nanomaterials including CNTs. This mechanism also originates from a deep analysis of the behavior of carbon nanomaterials in an oxidative medium. The mechanism proposed in Figure 5a is based on several consistent aspects. Fig. 5a1 schematizes the CNTs and the carbonaceous impurities. In this work, from combustion selectivity of the carbon impurities observed with and without chlorine (see for instance D1 and D3), it is obvious that chlorine plays the major role in preventing CNT from combustion by oxygen. Reactivity of CNTs with halogens or halogenated compounds has shown remarkable



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effects in carbon chemistry.61 For example, regarding fluorine, easiness62,63 and reversibility64,65 of fluorination of CNTs account for its extensive interest especially since fluorocarbons could be used in a lot of applications. Grafting of chlorine to large sp2-based carbon materials or carbon impurities has been shown to be less favorable than the formation of covalent C-Cl bonds on smaller and high quality CNTs.66,67In our samples, both DWCNTs and SWCNTs have a lower curvature radius (< 3 nm) compared to that within the non-nanotube species (> 5 nm), as observed by TEM. CNTs with a smaller diameter are thus expected to have a better affinity with chlorine (Fig. 5a2). Compared to the raw CNTs (Fig. 5b), Raman spectroscopy performed on the CNT sample for which the treatment was stopped just after the Cl2/O2 dwell (mD), on purpose, shows an increase of the D/G ratio compared to the raw CNTs (Fig. 5c). The intensity of the D band is decreased at the end of the Cl2/O2-process, as shown in Figure 5d and in agreement with Figure 4. We have not observed any G-band shift probably because of the too small amount of chlorine-containing functional groups present at the CNT walls. Such G-band shift due to doping effect by halogen was reported to be weakly pronounced or not visible after fluorine grafting.63,68 However, the observed D/G ratio for mD may be attributed to covalent functionalization of the CNT walls by chlorine (cf. also Supporting Information, Fig. S3). This hypothesis is also supported by AES of this CNT sample for which chlorine could be well detected69 (Fig. 6a and FTIR results, Supporting Information, Fig. S4). XPS analysis also revealed the presence of chlorine species at the surface of mD (Fig. 6b). Two components (each consisting of the 3/2 and 1/2 level with a spin-orbit splitting of 1.6 eV) were required to obtain a satisfactory fit of the Cl 2p core level spectrum. The low energy contribution with 2p3/2 component at ca. 197.5 eV corresponds to more electronegative chlorine atoms probably coming from metal. The high energy contribution with 2p3/2 component at ca. 199.9 eV is attributed to chlorine bonded to carbon70 in agreement with Raman spectroscopy. From XPS,


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chlorine atomic content was decreased of a factor of almost 2 between mD and D1, being as low as around 0.83 at.% in D1 (Supporting Information, Fig. S5).

Figure 5 a) Proposed mechanism for the selective removal of carbon impurities. 1) t = ti; 2) t  t1; 3) t1  t  t2; 4) t = tf, tx is the time sequence as defined in Fig.1. And typical Raman spectroscopy spectrum of DWCNTs at the raw state (b), DWCNTs collected just after the Cl2/O2 step (c) and DWCNTs at the end of the purification procedure (d). incident = 632.8 nm



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Figure 6 Auger (a) and XPS (b) analysis of mD.

Even if chlorine is only sparsely attached to the CNTs, its known power as a flame retardant leads to a favored carbon impurity combustion when oxygen is introduced in the reactor. This protective shield against combustion, which has been already suggested without any evidence38,42, leads to combustion of carbonaceous impurities by O2 at 𝑇𝑂2 while the CNTs are protected by Cl2 (Fig. 5a3). This protection induces a substantial improvement of removal selectivity between carbonaceous impurities and CNTs close to the combustion temperature (S1, D1). This beneficial effect is disappearing when 𝑇𝑂2is increased since carbon combustion becomes highly favorable and very fast (S2, S3 and S4). The well-known lability of the C-Cl bond certainly favors detachment of chlorine from the CNTs by heating (Fig. 5a4 and Fig. 5d).


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Based on the proposed mechanism, our method is versatile since purification occurs with optimum efficiency as oxygen is introduced at the combustion temperature of the samples for both DW- and SWCNT.

Conclusion This work proposes a solution to the long-standing issue of carbon impurities removal while avoiding excessive attack and consumption of CNTs. The challenge comes from the widely known too close chemical reactivity of carbon impurities and CNTs. This is an important finding since CNT purification cannot be avoided for a lot of applications and the existing purification methods are time and sample consuming. Our one-pot treatment consists of heating the sample under chlorine up to 𝑇𝑚𝑎𝑥 (around 1000 °C) and introducing oxygen at a given temperature 𝑇𝑂2 ≤ 𝑇𝑚𝑎𝑥. The best treatment is then obtained when 𝑇𝑂2 corresponds to the combustion temperature range of the CNT sample, as determined by thermogravimetric analysis. We have shown that chlorine plays an essential role by combining removal of metallic impurities and protection of the CNTs from combustion. Moreover, the efficiency of the proposed method has been demonstrated to be due to a substantial increase in the combustion selectivity of the carbonaceous impurities with respect to CNTs. The proposed mechanism appears then to be a powerful tool since it could be easily adapted to other source of carbon nanostructures with also other kind of metal impurities (Ni, Al…). Moreover, the present approach has the enormous advantage to produce, in a one-pot treatment, large volumes of purified CNTs. The developed treatment process has a hence high scale-up potential and it could open new opportunities for CNT applications.



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Acknowledgments The authors would like to thank Mr. L. Aranda and Mr. P. Franchetti for their help for TGA and Raman spectroscopy experiment, respectively. They also thank Prof. Dr. M. Dubois for the fruitful discussions about fluorine chemistry. Mr. P. Lonchambon is acknowledged for help with sample preparation. The authors thank the University of Lorraine for its financial support especially for Miss. N. Berrada’s Doctoral Fellowship. Auger experiments were performed using equipment from the TUBE—Davm funded by FEDER (EU) at IJL, ANR, the Region Lorraine and Grand Nancy. We acknowledge Mr. L. Pasquier and Prof. Dr. S. Andrieu from Institut Jean Lamour for performing the Auger experiments and analysis. We would like to thank the platform “Spectroscopies et Microscopies des Interfaces” (Laboratory of Physical Chemistry and Microbiology for Materials and the Environment, LCPME, Nancy, France) and Mr. A. Renard, Dr. M. Mallet (LCPME) and Dr. M. Dossot for XPS and absorbance analyses. This work was partly supported by the PRC CNRS/RFBR grant No 1023.

Supporting Information Available: Experimental conditions for the performed treatments are given for both DWCNTs and SWCNTs. Stability against combustion from TGA under dry air is discussed after applying an annealing program under helium. Carbon impurity in SWCNT samples was investigated by UV-NIR absorbance spectroscopy. Chlorine grafting to CNT walls was evidenced by RBM analysis from Raman spectroscopy and with FTIR. XPS wide scans are shown for rD, D1 and mD. This material is available free of charge via the Internet at http://pubs.acs.org.


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Selective elimination of both metallic and carbon impurities by a gas-phase purification method 214x103mm (150 x 150 DPI)

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Protecting Carbon Nanotubes From Oxidation for Selective Carbon

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